There is a need for sensors which detect hydrodynamic flow conditions, as well as fluid density conditions and variations, in a manner that reflect true conditions in that the sensor structure itself does not interfere with fluid flow at the location being monitored. For example, in monitoring fluid flow conditions over an airfoil, it is advantageous from both testing and fluid control purposes to know how the fluid environment is interacting with the airfoil at a specific, but perhaps fleeting moment. This is because slight variations in fluid dynamic conditions can over even very short periods of time give rise to situations of considerable interest. This is not only an issue in aerodynamics, but is also of great interest in medical applications where the flow of blood through the circulatory system is monitored. This is because circulating blood is constantly changing in pressure, velocity and density as a myriad of physiological conditions react with the blood stream.
The ability to detect fleeting changes in fluid flow conditions is useful in many other situations, such as but not limited to, the flow of fluids in hypersensitive chemical processing plants and the flow of gases through systems such as air conditioning ducts and gas scrubbing systems. There are many situations in which maintenance of laminar fluid flow is important, such as air induction systems of internal combustion engines, wherein laminar flow of combustion air is important to maximize efficiency in order to reduce pollutants and fuel consumption.
The need for non-intrusive, i.e., small, fluid sensors is also apparent in the marine industry in which vehicles are propelled through two fluids simultaneously, i.e., air and water, which fluids are separated by a very complex interface. Maximizing the efficiencies of hydrodynamic surfaces on marine vessels requires knowledge of what occurs or is occurring at boundary layers directly adjacent to or perhaps even perhaps within skin structure defining the surfaces.
Further examples of the need to understand and thereby control fluid flow over surfaces are exemplified by the need of next-generation lighter-than-air cargo and passenger air ships and by competition to improve the effectiveness of sails on racing boats such as America's Cup yachts.
Currently, the complexities encountered when attempting to comprehend boundary layer flow are perhaps best understood through three scalar partial differential equations that describe conservation of momentum for motion of a viscous, incompressible fluid. These complexities are frequently expressed mathematically in one complex expression, which relates fluid density, fluid velocity, fluid pressure, body force, and fluid viscosity. This equation has few mathematical solutions. Thus, a sensor which effectively monitors boundary layer conditions would be of considerable assistance in coping with, and effectively functioning within, an area of technology that has historically been extremely difficult to comprehend due to its complexity.